An improved method for use with an open-path fast Fourier infrared spectrometer performs real-time, spectral alignment on measured interferograms to reduce measuremental errors. The improved method includes the step of selecting a plurality of water-vapor lines in a defined spectral region and comparing the centerline of these measured water vapor lines to a reference library. From these comparisons, the spectrometer calculates correction factors to apply to the spectrometer bandwidth. The improved spectrometer performs transform functions on selected segments of the spectrometer bandwidth which introduce integer- continuous corrective shifts on subsequently measured interferogram data.
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